The present invention relates to graft copolymers and compositions suitable for cathodic electrodeposition of polymeric coatings. Furthermore, it relates to a method of cathodic electrodeposition of such graft copolymers.
It is known that organic coatings can be electrodeposited either on an anodically-charged conducting substrate or on a cathodically-charged substrate. Although most of the earlier work in electrodeposition was done with anodic deposition, that type of process has certain disadvantages. Anodic electrodeposition is normally done in a coating bath having a basic pH. The pH decreases at the surface being coated, creating conditions which, when combined with the electrolytic action of the coating bath, can cause the dissolution of substrate metal ions and their subsequent deposition in the coatings being formed. This can be a source of staining and diminished corrosion resistance. Also, electrolysis tends to attack preformed phosphate coatings on the anode. Furthermore, oxygen formed at the anodic substrate being coated can cause a variety of difficulties such as degradation of coatings by oxidation.
Electro-endoosmosis tends to expel water from anodic coatings being formed, leading to low water retention with about 85-95% solids in the coatings. This is an advantage over cathodic coating in which this phenomenon would not be expected to be helpful. (Parts and percentages herein are by weight except where indicated otherwise.)
Cathodic electrodeposition has developed more slowly, due in part to the acidic pH needed for the bath. Also, water tends to be drawn into the coatings and held there, along with acid residues from the bath. It is apparent that this can lead to difficulties in the coatings. In contrast to the oxygen formed at anodes in anodic electrodeposition, hydrogen is formed at the cathode in cathodic electrodeposition. Even though this hydrogen can cause pinholes in coatings, it, of course, does not cause oxidative film degradation.
Processes and compositions for the cathodic electrodeposition of paints are described in U.S. Pat. No. 2,345,543--Wohnsiedler, et al. (1944), which uses a cationic melamine-formaldehyde resin, and in U.S. Pat. No. 3,922,212--Gilchrist (1975), among others. Gilchrist is directed to a process for supplementing the bath composition with a make-up mixture of materials containing an ionizing acid that is not consumed at as fast a rate as the resin. The acid is present in the make-up at lower concentrations than are used in the bath, so as not to build up the concentration of the acid in the bath. Gilchrist uses particular amino-alcohol esters of polycarboxylic acids and discloses that acrylic polymers can be codeposited with zinc phosphate from solution on a cathodic substrate at low pH's such as 2.7 with phosphoric acid or volatile organic acids as the ionizing acid. Higher pH levels would be desirable for minimizing corrosion of coating equipment, especially if volatile nonpassivating acids are used for solubilization instead of phosphoric acid.
Two U.S. patents dealing with nitrogen-based copolymers and their cathodic electrodeposition are U.S. Pat. No. 3,455,806 and U.S. Pat. No. 3,458,420, both to Spoor, et al. (1969). Cathodic sulfonium systems are described by Wessling et al. on pages 110-127 of "Electrodeposition of Coatings," Ed. E. F. Brewed, American Chemical Society (1973).
Electrodeposition processes have been frequently described in the literature. Two useful reviews of the technology are: "Electro-painting Principles and Process Variables," Brower, Metal Finishing, September, 1976, p. 58; and "Coatings Update: Electrocoating," Americus, Pigment and Resin Technology, August, 1976, p. 17.
Dual cure cathodic electrocoating compositions are described in U.S. Pat. No. 4,070,258--McGinniss (1978) using a polymer with pendant tertiary amine groups, pendant mercaptan groups, an ethylenically-unsaturated carbonyl crosslinking agent and a photosensitizer. The deposited compositions are cured by using both heat and ultra-violet radiation. U.S. Pat. No. 4,066,523--McGinniss (1978) is similar except it uses a bismaleimide crosslinking agent.
U.S. Pat. No. 4,066,525--Woo and Evans (1978) provides a cathodic electrocoating process using an aqueous dispersion of (a) an ionized reaction product of an epoxy resin and a monoamino alcohol or phenol, said reaction product also having a pendant hydrocarbon group, (b) an acid-functional aminoplast resin, and (c) a water-soluble acid solubilizer.
Cathodic electrocoating systems are based on alkaline cationic resins that are solubilized or dispersed in water with the aid of an acid. In order to minimize corrosion of tank construction materials, it has been the aim of the industry to develop technology that will result in cathodic systems that are stable in water at close to neutral pH. This could be achieved by incorporating in the cationic resin strong alkaline functionality such as quarternary ammonium salts, primary or secondary amines, or combinations thereof and solubilizing the resin in water by neutralizing the amine with a weak acid--usually an organic acid. Because of the high basicity of the deposited film, one of the major problems in the development of such cathodic systems has been to obtain adequate cure, using conventional aminoplasts as crosslinking agents, at relatively low temperatures of 150.degree.-175.degree. C., which are important for various applications such as in the automotive industry. The difficulty in obtaining adequate cure is caused by the fact that the crosslinking of conventional aminoplasts of the melamine benzoguanamine or urea formaldehyde type requires acid catalysis and is strongly inhibited by a basic environment. For this reason, technologies have been developed for cathodic systems that use partially or fully blocked isocyanates as the curing agents. The crosslinking of isocyanates is base-catalysed and requires a basic environment. Isocyanate crosslinking has several shortcomings, including the need for usually high temperature or catalysts to unblock the isocyanate, high cost, and toxicity of raw materials and possibly of oven effluent during bake. It has been felt that the cathodic electrocoating technology will be at a disadvantage if it is restricted to such mechanisms of cure.
None of the prior art provides a fully-satisfactory composition for cathodic electrocoating at nearly neutral pH with the ability to cure at relatively low temperatures and times, with or without a crosslinking agent.